Soft flexible polyurethane foams have traditionally been made by reacting one or more organic polyisocyanates with one or more polyols having a molecular weight in the range of from 400 to 12,000, in the presence of a catalyst, a blowing agent, and if required, auxiliaries and/or additives (foam stabilisers, flame retardants, liquifiers, cell-openers, etc.). A halogenated hydrocarbon, such as a freon, is commonly used as the blowing agent. Soft to very soft foams are prepared in this manner, with a (relatively) high to very high density, depending among other factors on the character of the polyol(s) (particularly the molecular weight thereof) and of the polyisocyanate(s).
Because the use of halogenated hydrocarbons as blowing agents is suspected to cause environmental and health problems (for instance, as a result of their role in promoting the deterioration of the earth's ozone layer), there is a growing need for a blowing agent that may partly or even entirely replace the halogenated hydrocarbon in standard foam formulations. However, the use of substitute blowing agents should not result in a sacrifice in the quality of the product, or demand significant changes in the current processes and facilities for foam preparation.
The present invention relates to the use in such a reaction process of a blowing agent which comprises water in admixture with one or more non-crosslinked, water-soluble, organic polyelectrolytes, e. g., a polyacrylate compound. Water-blown polyurethane foam preparation processes are well known in the art. Water reacts with isocyanate groups to evolve carbon dioxide. However, this prior art fails to disclose processes utilizing non-crosslinked, water-soluble organic polyelectrolytes.
A number of prior art references describe polyurethane foams which incorporate non-water-soluble, crosslinked polyelectrolytes. For instance, Great Britain Patent Specification 1,538,809 describes a process in which a polyurethane prepolymer is reacted with water in a mixture with a salt of a crosslinked carboxylmethyl cellulose. The crosslinked carboxymethyl cellulose salts are described as essentially water-insoluble absorbent fibers which are incorporated into the polyurethane foam to enhance its hydrophilicity. U.S. Patent 4,127,515 similarly describes reacting a prepolymer with water swellable, acidic carboxylic cross-linked polymers ("Carbopols"). Great Britain Specification 1,550,614 discloses composite absorbent materials incorporating cross-linked water-swellable starch particles in flexible polyurethane foams. Japanese Patent Specifications 57061076, 55151034 and 59004613 describe polyurethane foams containing water-absorbing resins such as sodium polyacrylate, carboxymethyl cellulose, polyvinyl alcohol, starch and the like.
In other prior art processes, water-soluble polyectrolytes have been added to aqueous emulsions of polyurethanes. The polyectrolytes have functioned to modify the physical properties of polyurethane products, rather than as blowing agents in a reaction. Japanese Patent Specification 60048704, for instance, teaches the addition of water-soluble high polymer (e.g., polyacrylic acid or sodium polyacrylate) to a polyurethane in an aqueous surfactant solution, for the preparation of a coating material useful in cosmetics. Great Britain Patent Specification 1,475,539 discloses coating techniques in which polyurethane polymers are dispersed into water together with thickeners such as sodium methyl cellulose, polyacrylates, alginates, and polyvinyl alcohols.
U.S. Pat. No. 4,127,515 also mentions the addition to polyurethane prepolymer/water reactions of small amounts of emulsion stabilizers, thickeners and/or protective colloids such as triethanolamine sulfate, carboxymethylcellulose, polyvinyl pyrrolidone, water insoluble soaps, and polyacrylic acid. U.S. Pat. No. 4,314,034 describes the manufacture of polyurethane foam sponges upon the mixing of a polyurethane prepolymer and water, in which suspending or thickening agents (particularly the Carbopol polyacrylic acid polymers) are used to suspend and lubricate polyester fibers added to give the sponges structural rigidity. European Patent Specification 220697 discloses that various alkali and alkaline earth metal salts of Bronsted acids act as foam modifiers for polyurethanes, enhancing mold release and green strength.